A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 41:1The Psalmist celebrates the blessedness of those who compassionate the poor, conduct strongly contrasted with the spite of his enemies and neglect of his friends in his calamity. He prays for God's mercy in view of his ill desert, and, in confidence of relief, and that God will vindicate his cause, he closes with a doxology. (Psa 41:1-13)
God rewards kindness to the poor (Pro 19:17). From Psa 41:2, Psa 41:11 it may be inferred that the Psalmist describes his own conduct.
poor--in person, position, and possessions.
psa 41:2shall be blessed--literally, "led aright," or "safely," prospered (Psa 23:3).
upon the earth--or land of promise (Psa 25:13; Psa 27:3-9, &c.).
psa 41:3The figures of Psa 41:3 are drawn from the acts of a kind nurse.
psa 41:4I said--I asked the mercy I show.
heal my soul--(Compare Psa 30:2). "Sin and suffering are united," is one of the great teachings of the Psalms.
psa 41:5A graphic picture of the conduct of a malignant enemy.
psa 41:6to see me--as if to spy out my case.
he speaketh . . . itself--or, "he speaketh vanity as to his heart"--that is, does not speak candidly, "he gathereth iniquity to him," collects elements for mischief, and then divulges the gains of his hypocrisy.
psa 41:7So of others, all act alike.
psa 41:8An evil disease--literally, "a word of Belial," some slander.
cleaveth--literally, "poured on him."
that he lieth--who has now laid down, "he is utterly undone and our victory is sure."
psa 41:9mine . . . friend--literally, "the man of my peace."
eat . . . bread--who depended on me or was well treated by me.
hath lifted up heel--in scornful violence. As David and his fortunes typified Christ and His (compare Introduction), so these words expressed the treatment he received, and also that of his Son and Lord; hence, though not distinctly prophetical, our Saviour (Joh 13:18) applies them to Judas, "that the Scripture may be fulfilled." This last phrase has a wide use in the New Testament, and is not restricted to denote special prophecies.
psa 41:10A lawful punishment of criminals is not revenge, nor inconsistent with their final good (compare Psa 40:14-15).
psa 41:11favourest--or tenderly lovest me (Gen 34:19), evinced by relief from his enemies, and, farther, God recognizes his innocence by upholding him.
psa 41:12settest . . . before thy face--under thy watch and care, as God before man's face (Psa 16:8) is an object of trust and love.
psa 41:13Blessed--praised, usually applied to God. The word usually applied to men denotes happiness (Psa 1:1; Psa 32:1). With this doxology the first book closes.